A model has been developed to accurately calculate the nuclide inventories of the target constituents of Inertial Fusion Energy (IFE) reactors such as HYLIFE-II. It can explicitly account for (1) the combined effects of activation during target implosion (by a high-amplitude flux) and while passing through the reactor chamber (by a low-amplitude flux); (2) decay during circulation in the primary coolant loop, after extraction from the coolant loop, and before re-insertion into the reactor chamber as a new target; (3) continuous extraction and feed-in of target material; and (4) replacement of part of the activation products by makeup materials. The solution strategy uses transition factors – the ratio of the amount of created nuclides to the initial amount – for each system component.